In contrast to a stereoscopic representation, a holographic reconstruction realises an object substitute, which is why the problems known in conjunction with stereoscopy, such as fatigue of the eyes and headache, do not occur, because there is generally no difference between watching a real scene and a holographically reconstructed scene. The eyes of an observer can be served in a time- or space-division multiplexed presentation of different video holograms which differ in parallax.
High-resolution, flat light modulators which comprise several millions of modulator cells and which are used as screens in video and TV devices or projectors are for example particularly suited as light modulator means. Suitable light modulator means can be, for example, so-called liquid crystal on silicon (LCoS) modulators, where optical modulator elements with electronic circuits are disposed on a substrate or chip, transmissive LCD panels or electro-mechanically controlled micro mirror systems, such as micro electro-mechanical systems (MEMS), which comprise a combination of mechanical elements, actuators and electronic circuits disposed on a substrate or chip.
A light modulator means achieves the larger light diffraction angles the smaller the distance between the centres of the modulator cells, i.e. the cell pitch.
A reconstruction system is known from the international publication WO 2004/044659, titled “Video hologram and device for reconstructing video holograms”, which employs for spatial light modulation a liquid crystal display (LCD) screen with conventional resolution, as used for television and video image representations. The reconstruction system comprises focussing means which are disposed between the illumination means and the light modulator, and it makes it possible to holographically reconstruct a scene with the help of a conventional liquid crystal display, which has a modulator resolution that is relatively low compared with other solutions used in video holography. The reconstructed scene is visible with great spatial depth and good resolution in a reconstruction space through a visibility region which lies near an eye position of an observer. A viewing angle which is determined by the diagonal of the liquid crystal display screen can be used for the holographic reconstruction.
When using a light modulator which was designed for conventional image representation in holography, it is disadvantageous that the light modulator exhibits a diffraction angle which is rather small for holography, due to a typical distance of approx. 200 μm between the modulator cells. This makes it impossible for a reconstruction to be watched simultaneously with both eyes through a single visibility region. For such small visibility regions which are limited by the resolution, the prior art reconstruction system has it that separate visibility regions are generated sequentially for each eye of the observer. This means that, while a video hologram is active, the system alternately directs a wave field which is modulated with hologram information at the observer eyes for a fraction of the period of a video hologram. This makes great demands on the working speed of the light modulator means.
The reconstruction system according to the publication WO 2004/044659 thus additionally discloses a possibility for directing and tracking the position of multiple visibility regions. In particular, the reconstruction system realises a mechanical or electronic offset of the light sources laterally to the optical axis of the system using moving mirrors or multiple differently positioned light sources for displacing the light source images which generate the visibility regions for perceiving the reconstruction. When the observer moves, the light sources are repositioned in space such that the visibility regions follow the observer eyes.
It is disadvantageous that in a large tracking range, substantial aberrations, which occur when light passes through the focussing means, adversely affect the reconstruction of the spatial scene. The aberrations occur because the light passes through the focussing means at different angles, depending on the eye position, in order to reconstruct a scene. Because the object light points are reconstructed by wave interferences of diffracted partial light waves, such aberrations can cause image errors of a type not known from conventional video image representations as a result of phase and runtime errors. For example, other partial light waves than intended when computing the hologram according to the holographic source signal can interfere and generate misplaced object light points or additional object light points compared with the original scene.
In the international publication WO 2006/119920, titled “Device for holographic reconstruction of three-dimensional scenes”, the applicant also discloses a device which uses at least one visibility region which is smaller than the modulator surface of the light modulator at an eye position for watching the reconstruction. In that system, an array of light sources which are capable of generating interference and which are arranged in a matrix illuminates the modulator surface, and the focussing means comprise a multitude of imaging elements, e.g. convex lenses, which are adjoined mechanically so to form a flat array of focussing means. Each imaging element of the array of focussing means is assigned to at least one light source which is capable of generating interference, so to generate a bundle of illumination units which jointly illuminate the modulator surface, where each illumination unit only passes through a sub-region of the modulator surface. The light sources which are capable of generating interference in the illumination units are positioned such that the imaging elements of the array of focussing means image their assigned light source to an eye position. In other words, each illumination unit transmits a partial light wave through a sub-region of the modulator surface and, after a separate modulation by the individual sub-regions, the partial light waves overlap so to form a common visibility region.
According to a continuation of the known solution, for directing and tracking the position of the visibility region to changing eye positions, the light sources are designed as a plane backlight, and an additional controllable modulator matrix with modulator cells which can be switched to a transparent mode, for example a so-called LCD shutter array, opens, depending on the current eye position which is detected by a position detection system, for each imaging element of the array of focussing means a point light exit for the light which is capable of generating interference and which is focused on the eye position by the imaging elements. This generates a pattern of modulator cells which are switched to a transparent mode. In the case of a lateral change of the eye position, the position of the visibility region will be adjusted in that a system controller laterally displaces the pattern of the modulator cells which are switched to the transparent mode accordingly. In the case of an axial change of the eye position, the system controller will modify the distances between the modulator cells in the pattern which are switched to the transparent mode. The mentioned publication also discloses the usage of a switchable light source array with discretely controllable point light sources in order to realise the described process of directing and tracking the light wave field.
However, it has shown that the process of directing and tracking the position of the visibility region by adjusting the propagation of the light wave field according to the described solution exhibits several disadvantages. On the one hand, only a small fraction of the light energy can contribute to the reconstruction if an additional switchable modulator matrix is used, while the additional switchable modulator matrix absorbs the majority of the light emitted by the light source array.
On the other hand, a discretely switchable light source array or an additional switchable modulator matrix would be necessary which had to exhibit a much higher resolution than the light modulator. It is extremely complicated to provide such a light source array or such a switchable modulator matrix.
Both solutions also have a further major disadvantage which is that the coherent light of the backlight passes through the imaging elements of the array of focussing means at differently inclined transmission angles. The inclination of the transmission angle depends on the observer position and, depending on the actual eye position, results in substantial aberrations which are extremely difficult to compensate due to their dynamic nature. Moreover, those aberrations vary among the individual partial light waves because the propagation directions of the partial light waves towards the current eye position also differ.
Besides the mentioned solutions, the applicant also describes in the international publication WO2006/119760, titled “Projection device and method for the holographic reconstruction of scenes” a holographic projection system which uses a micro display with a diagonal of a few centimeters as light modulator. The device generates a visibility region for an eye position, similar to the previously described solutions. However, in contrast to the preceding solutions, where the light modulator forms the optical system exit of the modulated wave field and where the maximum viewing angle for the reconstruction is defined by the diagonal of the light modulator, that solution realises a projection device. In that device, a focussing display screen defines the maximum viewing angle and additional optical expansion means expand the wave field which is modulated with holographic information to the size of the display screen. A first imaging means images a video hologram which is encoded on the light modulator in an enlarged manner onto a focussing display screen, which images a spatial frequency spectrum of the video hologram to an eye position. The publication further discloses at least one controllable deflection element which is disposed inside the device and which serves for tracking the observer window according to the actual position of the observer eye. Such deflection elements can be mechanical, electrical or optical elements. The deflection element can for example be disposed in the plane of the first imaging means in the form of a controllable optical element which deflects the modulated wave field like a prism. However, it is also possible to dispose the deflection element near the display screen. This deflection element then realises the effect of a continuously controllable prism and, optionally, also the effect of a lens. The observer window is thereby tracked laterally and, optionally, axially. However, because a deflection element near the display screen serves to deflect the already modulated and enlarged wave field without disturbing the phase structure and the set interference conditions for the reconstruction of the scene, a modulator cell array which shall realise said aim must exhibit additional features which cannot be found in the publication.
All reconstruction systems described above use light modulator means with a discrete cell structure and a cell resolution which is rather low for holographic applications. On the one hand, as is generally known, the discrete cell structure causes a periodic continuation of the holographic reconstruction in other diffraction orders of a diffraction interval, so that the visibility may be impaired. On the other hand, the mentioned distance between adjacent modulator cells results in a relatively small diffraction angle, so that in practice a diffraction order with a diagonal of a few millimeters up to few centimeters is available for an undisturbed visibility of the reconstructed scene. It thus makes sense to combine the system controller of such a device with a position detection and tracking module. That module directs with the help of wave tracking means the modulated light waves at the current eye position, adjusts the position of the visibility region according to the eye position and tracks the light wave field each time the eye position changes.
Moreover, the light sources need to be positioned mechanically or, if the light source position is controlled electronically, a high spatial resolution of the light source field needs to be provided. In that case, the array of light sources must comprise a multitude of point light sources for each imaging element of the array of imaging means.
A controllable electro-optical cell, a so-called electrowetting cell, is known from the international publication WO 2004/099847, titled “Electrowetting cell”. These cells take advantage of the capillary effect and electrowetting effect in order to modify the surface tension of liquids using electrostatic potential and so to control the optical refraction behaviour. An electrowetting cell basically comprises a capacitor which is filled between the electrodes with a hydrophobic liquid, such as an oil, and water, where one of the electrodes is coated with a hydrophobic material. Without an electric field being applied, the oil covers the coated electrode as a film, and with an electric field being applied, the water displaces the oil film, because the applied field compensates the polarisation of the dipoles in the water surface. The cell can realise electronically controllable optical lenses and prism elements with a surface area ranging from below one square millimeter down to a few square micrometers.
An autostereoscopic image display device according to the international publication WO 2004/075526, titled “Autostereoscopic display” emits image light points horizontally in a multitude of directions without a tracking device. The image display device has a backlight which emits collimated light which propagates through the image light points of an image representation device towards an array of optical deflection means with dynamically controllable deflection behaviour. The optical elements are in particular electrowetting cells which are used as controllable lenses, and which realise a dynamically adjustable beam controller. In order to avoid the image representation having to be tracked to the current eye position of observers, a system controller frequently modifies with the help of the controllable array of optical deflection means both the exit angles of the light and the image content of the image representation device during each period of the video image. Thereby, up to one hundred emission directions are served in each video period using a combination of space-division and time-division multiplex methods, said emission directions lying closely side by side horizontally thus forming image sectors, so that each observer eye sees video images which differ in parallax without the need of tracking. The optical deflection means thus pan the beams which are temporally differently modulated by the image representation device over the multitude of the image sectors which lie closely side by side. The publication does not disclose any technical means which would explain how the system controller can deflect a modulated wave field which is capable of generating interference with the help of the array of optical deflection means.
In contrast to the subject of the present invention, the international publication WO 2004/075526 relates to an autostereoscopic image display device which does not reconstruct object light points in a holographic manner as a three-dimensional arrangement in a viewing space. Instead of the reconstructed object light points, an autostereoscopic image display device displays two-dimensional images in the modulator plane, said two-dimensional images having the form of luminous image points which carry multiple image information for both observer eyes. That system uses neither light diffraction nor light interference. The dynamically adjustable beam controller is designed to deflect bundles of rays with incoherent light in a simple manner and does not make any demands on the conditions for mutual interference of the deflected light beams. The bundles of rays, which lie close to each other, are in particular not able to prevent light of parasitic diffraction orders from entering. Moreover, a non-linear transmission behaviour in the boundary zones of the electrowetting cells would affect the propagation of the modulated light waves which are capable of generating interference, and would substantially disturb the interference behaviour of the reconstruction system and thus the quality of the reconstruction.
The publication does not disclose any technical means which would explain how the system controller can deflect a modulated wave field which is capable of generating interference with the help of the array of optical deflection means and how the effects of parasitic diffraction orders can be circumvented.